KINGSTON, N.Y. -- The Ulster County District Attorney's Office will not file criminal charges in the alleged falsification of state standardized test scores at Zena Elementary School because the evidence is insufficient, District Attorney Holley Carnright said on Tuesday.

Carnright said a lengthy investigation by his office led to the conclusion that there was not enough evidence to present the case to a grand jury.

"We're convinced the tests were tampered with," but the investigation had reached a dead end, Carnright said.

The prosecutor said "about a half-dozen" people had access to the tests and they all cooperated with the investigation and passed lie detector tests. But Carnright noted that lie detector test results are neither "infallible" nor admissible in court.

Kingston school district Superintendent Paul Padalino, who came on board in January, said the district will, to the best of its ability, continue to investigate what happened to the spring 2010 tests.

"We really don't have any idea who's responsible," Padalino said, describing the situation as a mystery and saying he is not satisfied with an inconclusive outcome.

"The students, parents and school community at Zena deserve to have answers about what happened," the superintendent added.

James Shaughnessy, the president of the Kingston Board of Education, said school officials are no closer to knowing what happened now than they were in November 2010, when they brought the matter to the District Attorney's Office.

In major cheating scandals that have made national news in places like Atlanta and Philadelphia, scores were improved. At Zena, however, the scores appear to have been made worse, and the motive is not clear, Padalino said.

Typically, Zena students had among the highest scores in the district, but the 2009-10 reading and math scores at the school plummeted, particularly in the third and fifth grades.

The focus of the District Attorney's Office was whether there were grounds for criminal prosecution, Padalino said, but the district's focus now is on the integrity of future tests, of which he said he is confident. The superintendent described the "misadministration" of the tests, which he said showed an excessive number of erasures, as an isolated incident.

Additionally, Carnright said some students had unusally high scores on the tests, while others had unusually low ones.

Since then, Padalino said, new protections and security measures have been put in place, like the installation of safes in individual buildings and increased monitoring of tests by administrators.

If the school district does manage to crack the case, Padalino said, it is not too late to discipline the person or persons responsible.